Neurotransmitter release is highly sensitive to Ca++ influx into nerve terminals. Although Ca channels have been extensively studied in many systems, including neuronal somata, it has been difficult to study the properties of nerve terminal Ca channels that are coupled to transmitter release. This project is aimed at understanding the properties of the presynaptic Ca channels at the vertebrate neuromuscular junction using a preparation that offers opportunity to study directly presynaptic type Ca channels: Xenopus motoneuron synaptic varicosities on muscle cells in culture. The properties of Ca channels will be investigated with cell attached and perforated patch voltage clamp methods. Changes in number or type of channels in response to target cell inductive influences and maturation will also be studied. By recording simultaneously from neurite varicosities and muscle cells, the degree of coupling between different Ca channel types and transmitter release will be determined and release parameters will be correlated with properties of the Ca channels carrying the current. Once the channel types that are coupled to release and their pharmacology are known, similar manipulation will be performed on mature neuromuscular junctions to see if they have the same effect on release. In addition the mechanisms by which Ach release is modulated by substances such as adenosine, norepinephrine, substance P, met- enkephalin, alpha-MSH, and vasoactive intestinal peptide will be investigated.